The present invention relates to an apparatus for measuring the area or area factor of an image portion of an image-bearing member having image and non-image portions with different reflectivities.
As an example of such an apparatus for measuring the image area, there is an apparatus for measuring the area of that portion of a printing plate used in an offset press which is inked, that is, the area of the image portion. The purpose of measuring the area of the image portion of the printing plate is to cause an ink supply device to supply a quantity of ink corresponding to the area of the image portion, thereby making proper printing possible.
In an apparatus for measuring the area of an image portion of a printing plate as disclosed in Japanese Patent Early Publication No. 56-54310, a large number of photoelectric conversion elements are disposed in a line on a movable stage and a pair of fluorescent lamps are disposed on the movable stage with the photoelectric conversion elements interposed therebetween. A printing plate is placed below the stage. As the stage moves over the printing plate, the printing plate is optically scanned. The image area is measured on the basis of outputs of the photoelectric conversion elements responsive to reflected light from the printing plate.
The size of printing plates used in the offset printing is normally about 1.2 m.times.1.4 m. Accordingly, the size of the image area measuring apparatus must also match this size. In order to accurately measure the image area of image-bearing member of a fairly large area, it is necessary to uniformly illuminate the whole area of a region to be measured and to uniformly receive the reflected light from the whole area of the region.
In the above-described conventional apparatus linear fluorescent lamps are used as a light source. Therefore, it is difficult to maintain a stable and uniform illumination for a long period of time. And moreover, because a large number of photoelectric conversion elements are used, the uniform reception of reflected light is difficult due to non-uniformity in sensitivity of the conversion elements. At the time of area measurement, since the large-sized stage is moved, it is difficult to measure at high speed.